20W pure Class A amplifier driven by NE5532 in parallel

This circuit is provided by the user Aishanleshui. It seems to come from a Japanese audiophile.

There are many well-made power amplifiers abroad. Their output power is not large, but their sweet and elegant music is often unmatched by many high-power amplifiers.

Although the components of the amplifier introduced in this article are average and its output power is only 20W, its musical expression is extremely outstanding, especially for the reproduction of classical music.

【Circuit Principle】

The circuit is shown in Figure 6-1. Two independent operational amplifiers (NE5532) are used in the circuit of this machine to form two complete single-ended amplifiers. They all work in pure Class A mode and independently form full-wave amplifiers with excellent performance. The amplified signals are organically mixed at the output point, effectively reducing the odd harmonic distortion that is extremely harmful to the sound quality. The bipolar diodes (VT1 and VT2) of the excitation stage are used as current control devices to directly draw the required base current from the output end of the operational amplifier, which is a more ideal way of use. VT3 and VT4 are used as constant current source loads for VT2 and VT1 respectively, which ensures the stability of the whole machine and also makes the machine free from troublesome debugging procedures.

The VT1 and VT2 of the excitation stage and the two high-power transistors of the output stage form a cross-coupling mode. Due to the clamping effect between the working points of each diode, the stability of this circuit is extremely good, and there will be no impact current sound when the power is turned on. Another benefit of cross-coupling is that the excitation stage and the output stage respectively obtain the working current from the positive and negative power supply terminals, which is very beneficial to improving the common mode rejection ratio of the amplifier. The working current of the excitation stage is as high as 85mA, and the working current of the output stage is as high as 1.7A (two tubes in parallel). Since the current of this machine is very large, each transistor (including the excitation stage and the constant current source load transistor) must be added with a large enough heat sink during production, and the power transformer must have sufficient margin (recommended 150W). Since this machine is highly adaptable to the power supply, the power supply circuit only needs simple rectification and filtering. If conditions permit, a 1~2H inductor can be connected in series in the power supply circuit to obtain better results.